Archive | July 19, 2012

Sharp spines, strange sex

Micrathena schreibersi from Suriname [Canon 1Ds MkII, Canon 100mm macro, two speedlights Canon 580EX]

I know many arachnophobes; in fact I live with one. But being afraid of spiders of the genus Micrathena is about as rational as being afraid of a diamond brooch. These animals are jewelry come alive, harmless, and each more ornate and vividly colored than the other.

There are about 100 known species of Micrathena (Araneidae), most of which are found in tropical areas of Central and South America. Females of these beautiful spiders can barely walk on their own if accidentally knocked off their orb, where they spend their entire life suspended upside down. Because their venom is weak and they cannot run away from danger, they protect themselves by covering their bodies with hard, sharp spines. In one extreme case, females of Micrathena cyanospina have spines that can be nearly 50 mm long. This probably can turn the act of  swallowing of such a spider by a bird or a lizard into the animal’s last meal.

The biology of most Micrathena species is completely unknown, but several, including three that occur in the US, have been studied in some detail. What we have learned about them shows that Micrathena have a really strange sex life. To begin, males are tiny compared with the females, and often the female is ten times as large as the male. They also differ greatly in their appearance, with the females being colorful and sedentary, whereas the males are usually dull-colored and constantly on the move. Both sexes have double copulatory organs, and for some strange reason after the first copulation, when only one set is used, the male must dismount the female and seduce her again before the second copulation can take place. But a lot of things can go wrong during that time, and often another male moves in on the action, and wins the second copulation, which appears to be more significant in the insemination process.

Micrathena clypeata from Suriname [Canon 7D, Canon MP-E65mm macro, Canon MT-24EX twin light]

Micrathena cyanospina from Suriname [Canon 7D, Sigma 15mm f/2.8 EX DG, Canon 580 EXII speedlight]